1. Field of the Invention
The present invention relates to a lens drive mechanism in an optical disc player such as, for example, a compact disc (CD) player or a laser disc (LD) player or any other player of a type utilizing a beam of light to scan an information carrier medium.
2. Description of the Prior Art
It is well known that commercially available optical disc players make use of a lens drive mechanism or lens actuator for driving an object lens in two directions substantially perpendicular to each other, i.e., a focusing direction and a tracking direction. The focusing direction is a direction in which the objective lens is driven parallel to the optical axis of the objective lens so that a laser beam emitted from an optical system can be focused on an optical disc to form thereon a beam spot of 1 .mu.m in size while the tracking direction is a direction substantially perpendicular to the optical axis of the objective lens and in which the objective lens is moved laterally of the optical axis so as to compensate for change in position of the beam spot relative to a particular information bearing track in the form of a train of pits and lands defined on the optical disc, which change would occur as a result of a wobbling or eccentric motion of the optical disc. Control of the objective lens in these two directions is important in accomplishing a faithful recording or reproduction of information on or from the optical disc then rotating in one direction.
A typical prior art lens drive mechanism of the kind referred to above comprises a stationary unit including a support base having upright yokes perpendicular to the base, and permanent magnets fixedly carried by the base so as to confront the associated permanent magnets to thereby form respective magnetic circuits therebetween. The lens drive mechanism also comprises a movable unit including a generally tubular lens holder having an objective lens mounted thereon and carrying both a focusing coil externally around the lens holder and generally flat tracking coils cemented to respective portions of the focusing coil that are opposite to each other with respect to the optical axis. The movable unit is supported by the support base by means of upper and lower pairs of elongated spring elements for movement in the tracking direction and also in the focusing direction.
This prior art lens drive mechanism is so designed that, when the focusing and tracking coils are electrically energized as a result of flow of an electric current therethrough, electromagnetic forces are developed between the coils and the magnetic circuits to drive the movable unit, specifically the lens holder, in two directions substantially perpendicular to each other as is well known to those skilled in the art.
In this type of an lens drive mechanism requiring such a two-directional drive, it has been recognized that any unnecessary torsional resonance such as a rolling motion of the objective lens must be eliminated. For this purpose, the use has been suggested of a counterweight secured to a bottom portion of the lens holder to adjust the center of gravity of the lens holder in an attempt to eliminate any possible torsional resonance such as disclosed in, for example, the Japanese Laid-open Patent Publication No. 58-182140.
However, the use of the counterweight makes it difficult to render the lens drive mechanism, particularly the lens holder, to have a reduced axial dimension. The use of the counterweight also results in an increase of the weight of the lens holder, which in turn results in an increased of the electric power consumed by the lens drive mechanism.